This invention generally relates to the field of electric power conversion systems for conveying power between a direct current (DC) power source and an electric load circuit. More particularly, the present invention relates to an improved electrical circuit configuration for a chopper circuit which is particularly adapted for use with an inductive load such as the braking resistors of the type used in many rapid transit vehicles.
Rapid transit vehicles are typically powered by electric traction motors whose rotatable shafts are mechanically coupled to an axle/wheel set of the vehicle. A wayside conductor, generally located near the right-of-way along which the vehicle travels, provides the DC electric power for the motor. An electric power conversion system is used to condition the DC electric power supplied to the motor load circuits. If a DC motor is used, such a system will include an electric power "chopper" which is suitably controlled to vary the magnitude of load current and/or voltage. In the case of alternating current (AC) motors, the electric power conversion system will typically include an electric power "inverter" functioning as a DC-to-AC converter that is suitably controlled to vary the amplitude and frequency of the load voltage as desired. In either case, electric power flows from the DC source, via the wayside conductor, through the electric power conversion system and into the load during the "motoring" or "propulsion" mode of operation.
Such traction vehicles often depend upon electrical braking by the traction motors to assist mechanical or friction brakes in stopping the vehicle. In order to provide this electrical braking effort, the traction motors are electrically controlled to operate as electrical generators driven by the rolling wheels of the vehicle. Accordingly, in the "electrical braking" or "retarding" mode of operation, the traction motors convert the kinetic energy of the vehicle's inertia into electric power which flows in a reverse direction through the power conversion system, now serving, for example, as an AC-to-DC converter. Thus, the generated voltage appears as a DC voltage at the input terminals to the power conversion system. The method of disposing of this surplus electrical power depends upon the type of electrical braking being utilized.
In general, only two types of electrical braking are in common use: dynamic braking and regenerative braking. Dynamic braking is effected by connecting a dynamic braking resistance between the DC source terminals and dissipating the surplus power. This resistance receives current from the converter, transforms the electrical energy into thermal energy, and dissipates the resulting heat. Regenerative braking, on the other hand, is effected by returning the surplus power to the DC power source during the braking operation. Both dynamic and regenerative braking ability may be incorporated into the same vehicle control system, with an appropriate sensing apparatus for determining when to use dynamic braking and when to use regenerative braking. This combined braking system is commonly referred to as "blending". The desired blending of dynamic and regenerative braking can be accomplished in various different ways which are well known to persons skilled in the art. For example, U.S. Pat. No. 4,093,900-Plunkett utilizes a parallel array of separate braking resistors to perform the blending.
In many dynamic braking systems, a chopper circuit is used to modulate the amount of time during which the dynamic braking resistor is connected in the circuit. The chopper is essentially a controlled electronic switch connected in series with the braking resistor. The average value of the resistor current is thus regulated by varying the ratio of the amount of time the switch is closed, or the switch's "on time", to the amount of time it is open, or its "off time". In modern solid state systems, the chopper includes a power thyrister device connected directly in series with the braking resistor. A silicon controlled rectifier (SCR) or a gate turn-off device (GTO) are two types of thyrister devices which can be used for this electronic switch. The choice of thyrister would depend upon the type of pulse-width modulation (PWM) circuitry used to control the average magnitude of current in the resistor, as well as other circuit parameters.
When a forward voltage is supplied to a thyrister, and it is turned on by means of its gate current, the conduction of anode current across the device junction commences in the immediate neighborhood of the gate connection, and spreads from there across the whole area of the junction. This conduction spreads across the cathode area of the thyrister at a rate of about one centimeter per 100 microseconds. If the rate at which the anode current increases is much greater than the rate at which the conduction area increases, there will be a high power density in the conduction area. This results in a local "hot spot" formed in the neighborhood of the gate connection due to the high current density in that part of the junction that has begun to conduct. Such localized heating may produce excessively high temperatures, may possibly result in permanent damage to the thyrister, and may ultimately result in failure of the dynamic braking circuit. For this reason, the maximum rate of rise of anode current, termed "di/dt.sub.MAX ", is usually specified by the manufacturer of the thyrister device. Typical values for di/dt.sub.MAX are 30-200 amperes-per-microsecond (A/.mu.sec) for phase-controlled SCR's, and as high as 800 A/.mu.sec for inverter GTO's.
In order to limit the rate of current rise through the thyrister, an external inductor is typically connected directly in series with the thyrister to be protected. This series inductor, termed a "di/dt reactor", need only introduce a small amount of inductance in the circuit, and should not saturate or go into a low impedance state in less than the turn-on time of the thyrister device. For example, a 5 microhenry (.mu.H) inductor, saturating only after a few microseconds, would sufficiently limit the rate of rise of current in the series combination, thus reducing the series di/dt, and thereby protecting the thyrister device.
Although the inductance value of the di/dt reactor need only be a few microhenrys, the power rating of the di/dt reactor often presents a problem for high-current electric power conversion systems. For example, a rapid transit vehicle operating from a power source of 600 volts DC would have a typical average current of approximately 500 amperes RMS through the series combination of the di/dt reactor and the chopper thyrister. To protect against current spikes, a di/dt reactor having a maximum current rating of 1000 amperes is often used. However, the size of such a di/dt reactor is rather large and heavy, i.e., typically on the order of 10.times.10.times.6 inches at approximately 40 pounds. Moreover, such an electrical component can be very expensive, particularly when multiple resistance braking circuits are used in a single electronic power control system.
A need, therefore, exists for an improved chopper circuit for dynamic braking of an electric power conversion system, which provides for the reduction of the size, weight, and cost of the di/dt reactor.